1. Field of the Invention
The present invention relates to a scintillator crystal that emits light by irradiation with radiation and a radiation detector including the scintillator crystal.
2. Description of the Related Art
In an X-ray computed tomography (CT) scanner used in, for example, clinical practice, a scintillator receives X-rays passing through a subject, and light emitted from the scintillator is detected by photodetectors.
Japanese Patent Laid-Open No. 2012-131964 (US counterpart: U.S. Patent Application Publication No. 2012/0292516) discloses a scintillator crystal (phase-separated scintillator crystal) having a phase-separated structure including a plurality of columnar crystals (cylindrical phase) and a crystal phase (matrix phase) around the columnar crystals. In the scintillator crystal, a material constituting the cylindrical phase differs in refractive index from a material constituting the matrix phase. This imparts a light-guiding function to the scintillator crystal itself, thus inhibiting light crosstalk.
Japanese Patent Laid-Open No. 2012-131964 discloses that in the case of a scintillator crystal in which the amount of light emitted from the matrix phase is larger than that from the cylindrical phase when the phases are irradiated with radiation, the matrix phase may be composed of a material having a higher refractive index than the cylindrical phase. The reason for this is that in the case where the matrix phase is composed of a material having a higher refractive index, it is possible to more efficiently confine light generated in the matrix phase, thereby enhancing light-guiding properties.
In this specification, among phases constituting a phase-separated scintillator crystal, a phase composed of a material having a high refractive index is referred to as a “high-refractive-index phase”, and a phase composed of a material having a low refractive index is referred to as a “low-refractive-index phase”. The magnitude of the refractive index is a relative scale. When a phase is composed of a material having a higher refractive index than a material constituting a low-refractive-index phase, the phase is regarded as a high-refractive-index phase. In general, a lower refractive index ratio of the low-refractive-index phase to the high-refractive-index phase (low-refractive-index phase/high-refractive-index phase) may result in higher light-guiding properties because light can be confined in the high-refractive-index phase.
Japanese Patent Laid-Open No. 2012-131964 discloses NaCl—CsI and KCl—CsI as examples of a scintillator crystal in which the amount of light emitted from the matrix phase is larger than that from the cylindrical phase when the phases are irradiated with radiation and in which the matrix phase is a high-refractive-index phase.
However, it was found that in the scintillator crystals disclosed in Japanese Patent Laid-Open No. 2012-131964, a scintillator crystal having a lower refractive index absorbs a smaller amount of radiation and that the absorption of radiation is not sufficient, depending on the energy of radiation, in some cases. In the case where the scintillator crystal insufficiently absorbs radiation, a light-receiving device located downstream of the scintillator crystal can be damaged by radiation.